1. Field of the Invention
The present invention relates to a fine pattern forming material used when semiconductor devices and integrated circuits are made by forming a pattern thereon using an electron beam, and to a fine pattern forming method using such material.
2. Description of the Prior Art
Conventionally, in the manufacture of ICs and LSI circuits, patterns are formed by photolithography using ultraviolet rays. To cope with a requirement to make devices smaller in size, a numerical aperture of a stepper lens is increased and a light source having a shorter wavelength is used, which results in a drawback in that a focus depth is shortened. Further, as a pattern of LSI devices is made finer in size and ASICs are put into production, electron beam lithography has been used
Positive type electron beam resists are indispensable to form a fine pattern by electron beam lithography. Polymethyl methacrylate (PMMA) is known as a resist having the best resolution among them, but it has a drawback of low sensitivity. Thus, recently, there have been numerous reports relating to improving the sensitivity of positive type electron beam resists, in which reference is made to positive type resists such as polybutyl methacrylate, a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methyl methacrylate and isobutylene, polybutene-1-sulfon, polyisopropenyl ketone, fluoro polymethacrylate, and the like. These resists, which aim at an improvement in sensitivity, are arranged such that an electron withdrawing group is introduced to the side chain or an easily degradable bond is introduced to the principal chain thereby to enable an electron beam to easily scissor the principal chain, but they cannot sufficiently improve both resolution and sensitivity. Further, since they do not have sufficient dry etching resistance and heat resistance, they are difficult to be used as a mask for dry etching, and thus a field to which they are applicable is limited.
Negative type electron beam resists using cyclized rubber as a base have inferior adhesion to a substrate, thus suffering a drawback in difficulty in forming a coated uniform layer of high quality without pin holes on the surface of the substrate and inferior heat stability and resolution. Therefore, conventionally, various improvements have been made to the negative type electron beam resists. For example, negative type electron beam resists such as polyglycidyl methacrylate, chloro methyl polystyrene, chloro methyl .alpha.- methyl polystyrene, polymethacrylate maleic acid ester, polystyrene chrolide, a copolymer of glycidyl methacrylate and ethyl acrylate, and the like are reported. These resists, which aim at an improvement in sensitivity, are arranged such that an epoxy group easily reactive to electrons and chlorine atoms are introduced thereby to enable an electron beam to easily produce a radical to cause a cross-linking reaction, but do not have sufficient resolution and heat resistance. Although an organic solvent is needed to develop these negative type resists comprising a rubbery thermoplastic polymer using a cyclized rubber or polyisoprene as a base, the resists on which a pattern is written may be swelled in the organic solvent developer while they are developed. As a result, the resolution of the pattern is lowered, and sometimes the pattern is distorted and unable to be used. In addition, the organic solvent developer is harmful to the environment and health and further is not desirable from the view point of the igniting property thereof.
Electron beam lithography has such drawbacks as an adverse effect to a pattern accuracy caused by the inferior dry etching resistance and heat resistance of electron beam resists and a proximity effect due to forward and backward scattering of electrons, and an adverse effect to a pattern drawing due to charging by incident electrons, and the like. A multi-layer resist, the function of which is shared by its pattern forming layer and planarizing layer, is very effective to compensate for these drawbacks. FIGS. 3A to 3D are diagrams explaining a multi-layer resist process applied to the electron beam lithography. To suppress the proximity effect, an organic polymer film is applied to a thickness of 2 to 3 microns as a bottom layer 31 and subject to a heat treatment (FIG. 3A). Further, an inorganic film of SiO.sub.2 or the like, or an inorganic polymer film of SOG (spin on glass) or the like is applied thereon to a thickness of 0.2 micron as an intermediate layer 32, and an electron beam resist is applied to a thickness of 0.5 micron as a top layer resist 33. An aluminum film 34 of about 100 Angstroms is deposited thereon to prevent charging (FIG. 3B). After a pattern is written by an electron beam, the aluminum film 34 is removed with an alkaline solution, then a development is carried out to obtain a resist pattern 33P (FIG. 3C). Next, the intermediate layer 32 is dry etched using the resist pattern 33P as a mask, and further the bottom layer 31 is dry etched using the intermediate layer 32 as a mask (FIG. 3D). The use of the above multi-layer resist process enables a fine pattern to be formed at a high aspect ratio. In the multi-layer resist process in which the aluminum film is deposited, however, manufacturing processes are more complex and have such problems as contamination, an increase in dimensional shift caused when a pattern is transferred, and the like, and thus this process is not applicable to actual use.
As described above, although the multi-layer resist process using an aluminum film is an effective method, it has the problem of complex manufacturing processes, contamination of aluminum, and dimensional variation of the resist when the pattern is transferred.
Further, a multi-layer process not using an aluminum film has a problem of charging, which is a phenomenon wherein incident electrons are stored in a resist, an intermediate layer or a bottom layer, all being insulators. The charging effect causes a serious problem such as deterioration of a field butting accuracy and overlay accuracy, and the like in the electron beam lithography. In addition, the charging phenomenon is also observed in a single layer resist and causes the deterioration of a field butting accuracy and overlay accuracy, as in the three-layer resist. More specifically, incident electrons scattered in a resist in the electron beam lithography stay in a region at a depth of 1 to 1.5 microns from the surface of the resist and a charge is stored in this region. It is supposed that the stored charge causes an electron beam to be curved thereby to deteriorate the field butting accuracy and overlay accuracy.
When a pattern is written by an electron beam, a thick bottom layer must be applied, because a pattern accuracy is greatly affected by the proximity effect. Thus, a silicon containing resist, inorganic resist, and the like, which not only serve as a mask for the bottom layer but also serve as a resist layer, have been developed. They include a resist having a siloxane bond coupled to the principal chain thereof, ladder type polysiloxane, a chalcogenide glass type inorganic resist and the like, but cannot yet sufficiently improve dry etching resistance and also has inferior sensitivity and resolution. Thus they are far from achieving practical use. Since these resists employ an organic solvent as a developer, they have a large variation in sensitivity and dimension, a smaller process margin, and a problem of environmental pollution and the like.
Since a charging phenomenon is also caused in the two-layer resist process, as in the three-layer resist process, an aluminum film is deposited on a resist to prevent the occurrence of charging. Further, the resist process using an aluminum film has a problem in that a novolac type resist using an organic alkaline solution as a developer cannot be used, because an alkaline solution is used to remove the aluminum film.
The present inventors have developed highly sensitive conducting electron beam resists and a fine pattern forming method using the resists to solve these phenomena.